The objective of the proposed reseach is to correlate the architecture of photosynethetic membranes as visualized by freeze-fracture and freeze-etch electron microcopy with their functional organizaton at the supramolecular level. To this end the structural parameters of control and experimentally modified thylakoids of higher plants and of bluegreen algae will be both qualitatively and quantitatively evaluated and findings related to biochemical and physiological observations. Particular emphasis will be placed on elucidating the structure and composition of the light-harvesting chlorophyll a/b protein complex, its association with photosystem 2, and its role in thylakoid membrane stacking.